Numerous methods are known for the execution of electrophotography. At a general level, using a photoconductive material an electrostatic latent image is formed on an electrostatic latent image-bearing member (also referred to as a “photosensitive member” below) by various means. Then, a visible image is made by developing this electrostatic latent image with toner; as necessary the toner image is transferred to a recording medium such as paper; and a copied article is obtained by fixing the toner image on the recording medium by, for example, the application of heat or pressure. For example, copiers and printers are image-forming apparatuses that use such an electrophotographic procedure.
Previously, these printers and copiers were connected in networks and such printers were often tasked with printing from a large number of people. However, the modalities of use have grown increasingly diverse in recent years, and, for example, personal computers (PCs) and printers are also located locally outside the office and its normal environment, i.e., in high-temperature, high-humidity environments or low-temperature, low-humidity environments, and situations in which a task or activity is accomplished by printing an image are also on the increase. As a consequence, smaller size, high durability, and the ability to adapt to a wide range of environments are strongly desired in a printer.
A magnetic monocomponent development procedure using a magnetic toner (also referred to below simply as toner) is preferably used in relation to downsizing and high durability. When the environmental adaptability is more closely considered, the humidity presents itself among environmental factors as a factor that has a major influence on electrophotographic technology. The humidity contributes to quality variations in the development step as it has an effect on the amount and distribution of toner charge, and meanwhile it also has a major effect on the transfer step.
Considering problems related to the transfer step more closely, transfer defects are an example of image defects that are realized when there are problems during transfer. In the transfer step, the toner on the electrostatic latent image-bearing member is subjected to a transfer bias and is transferred onto the recording medium by electrostatic attraction. At this point, toner may remain on the electrostatic latent image-bearing member without undergoing transfer and the toner layer may undergo disturbances during transfer and defects and nonuniformity on the image may be produced as a result. These are called transfer defects. A discharge phenomenon—which can occur between the electrostatic latent image-bearing member and the transfer material due to the large bias being applied between the electrostatic latent image-bearing member and the transfer material—is a cause of transfer defects. When discharge occurs, the toner becomes an inversion component without maintaining the original amount of charge and undergoes re-transfer to the electrostatic latent image-bearing member. Due to this, the toner remaining on the electrostatic latent image-bearing member increases and the image may be disturbed and white voids may be formed.
In order to improve the transferability, countermeasures have been pursued to date through the external addition of a magnetic body while maintaining the flowability (Patent Literature 1, Patent Literature 2). However, the effects are inadequate in a high-humidity environment, in which discharge readily occurs.
On the other hand, toners have been disclosed that have sought to solve problems by focusing on the release of external additives (refer to Patent Literatures 3 and 4), but toner transferability again cannot be regarded as adequate in these cases.
Moreover, Patent Literature 5 teaches stabilization of the development—transfer steps by controlling the total coverage ratio of the toner base particles by the external additives, and a certain effect is in fact obtained by controlling the theoretical coverage ratio, provided by calculation, for a certain prescribed toner base particle. However, the actual state of adhesion by external additives is substantially different from the value calculated assuming the toner to be a sphere, and this theoretical coverage ratio has little effect with regard to the transferability in a high-humidity environment, which is the problem identified above, and improvement has thus been required.